The present invention relates to devices and methods for generating a drop of a liquid, e.g. for dosing systems for the dosage of small amounts of liquids.
The dosage of small amounts of liquids is widespread in many fields of application from inkjet printer to the production of microarrays, and different methods are used. Such methods are described, for example, in the following expert publications: P. Koltay, G. Birkle, R. Steger, H. Kuhn, M. Mayer, H. Sandmaier and R. Zengerle, “Highly Parallel and Accurate Nanoliter Dispenser for the High-Throughput-Synthesis of Chemical Compounds”, 2001, pages 115-124, in the following referred to as [1], P. Koltay, B. Birkenmaier, R. Steger, H. Sandmaier and R. Zengerle, “Massive Parallel Liquid Dispensing in the Nanoliter Range by Pneumatic Actuation”, H. Borgmann, Ed. Bremen: 2002, pages 235-239, in the following referred to as [2], and P. Koltay and R. Zengerle, “Non-contact nanoliter & picoliter liquid dispensing”, in Proceedings of the 14th International Conference on Solid-State Sensors, Actuators and Microsystems (Transducers & Eurosensors '07) Lyon, France: 2007, pages 125-129, in the following referred to as [4].
New fields of applications are constantly evolving. One of the most recent fields of application is three-dimensional printing, in particular for building prototypes. Thereby, on the one hand, binder can be printed on thin powder layers, or the building medium can be dispensed directly in liquid form and cured at the target. The latter can also be performed with melting, for example of polymers or metals, which are then cured by cooling at the target. Thereby, for example, printed circuit boards (PCB) can be printed directly.
In dosing systems, the following two dosing mechanisms can basically be distinguished:
contact dosage
non-contact dosage
In contact dosage, a media-carrying tool comes so close to the target area that a liquid drop of the medium at the tip of the tool comes into contact with the target area. By the adhesive forces between liquid and target area, part of the liquid remains on the target area when the tip moves away again.
In non-contact dosage, by introducing kinetic energy, a drop is ejected from a reservoir, frequently by means of a nozzle, and accelerated towards the target area. When impinging on the target area, it adheres again due to the adhesive forces.
The advantage of non-contact dosage is that smaller drops can be deposited on a target area from a certain distance. In contact dosage, the tool, frequently a needle, has to be brought so close to the target area that the drop touches the same, hence the smallest distance is approximately in the range of the drop diameter. Miniaturization necessitates, on the one hand, smaller needles, since the same have to be smaller than the drop to be generated, and, on the other hand also decreasing distances. The needles are constantly at a high risk of being damaged. This risk is even increased when dosing has to be performed in areas that are not topographically even.
In non-contact dosage, again, different methods can be distinguished:
open-jet tear-off
inertia-driven single drop dosage
shock wave method
pre-dosing method
In open-jet tear-off, a continuous liquid jet is generated out of a small nozzle. Due to energetic conditions, as they are described, for example in the expert publications by P. G. de Gennes, F. Rochard-Wyart and D. Quere, Capillarity and Wetting Phenomena: Drops, Bubbles, Pearls, Waves. New York: Springer, 2003 in the following referred to as [4], and L. Rayleigh, “On the Instability of Jets”, in Proceedings of the London Mathematical Society 1878, in the following referred to as [5], the jet disintegrates into drops of the same size and interval after a certain length.
When the resulting continuous stream of drops is not desired, the jet and hence the drops can be electrostatically charged within the nozzle and derived in the further course by applying electric fields and positioned, for example, on the target area. The disintegration into single drops is increased by the surface tension of liquid and decelerated by the viscosity of the liquid. Hence, this is also the disadvantage of the method. The jet disintegration depends heavily on the viscosity, such that the same no longer functions in a feasible manner with highly viscous media.
In the inertia-driven generation of single drops, the liquid in a nozzle is accelerated by means of a pressure pulse. This introduces kinetic energy into the liquid. If the same is large enough, a liquid drop tears off after the termination of the pressure pulse. Thereby, at first, part of the liquid is pressed out of the nozzle. This part is drawn back into the nozzle due to the surface tension and the negative pressure resulting in the nozzle when the pressure pulse is terminated. The inserted inertia energy stabilizes the drop at first outside the nozzle. On the one hand, the surface tension draws the drop back, but results, on the other hand, in instability of the resulting constriction as in jet decomposition, which can result in a pinch-off of single drops. When this pinch-off takes place before the outer drops change their direction of motion, a drop tear-off from out of the nozzle takes place still with finite speed.
Thereby, single drops with diameters in the order of the nozzle radius can be generated, as described in the expert publication by T. Lindermann, “Droplet Generation—From the Nanoliter to the Femtoliter Range”. PhD Dissertation, Institut für Mikrosystemtechnik (IMTEK) Lehrstuhl für Anwendungsentwicklung, Fakultät für Angewandte Wissenschaften Albert-Ludwigs-Universität Freiburg, 2006, in the following referred to as [6].
Another disadvantage is that the pinch-off of the drop becomes slower with increasing viscosity, and the functionality with respect to drops that are as small as possible is limited by the nozzle radius.
In shock wave methods, an acoustic shock wave is generated in the nozzle, which moves towards the end of the nozzle and there also tears off a drop by inertia effects and accelerates the same away from the reservoir. An advantage of the method is the option of generating drops that are smaller than the nozzle diameter. Additionally, the direction of the drop flight does not have to correspond to the nozzle main axis, but corresponds rather to the direction of the shock wave inside the nozzle.
One option of influencing the drop size of a drop to be dosed is pre-dosage within the nozzle system. There, a defined nozzle part is pre-filled, for example by capillary forces and then completely discharged by a subsequent pressure pulse. Thereby, a single drop is formed, which moves towards the target area. An advantage of the method is that the drop size essentially only depends on the nozzle geometry. Any amount of energy can be introduced, such that media of diverse surface tensions and viscosities can be dosed. See, for example expert publications: R. Steger, B. Bohl, R. Zengerle and P. Koltay, “The dispensing well plate: a novel device for nanoliter liquid handling in ultra high-throughput screening”, Journal of the Association for Laboratory Automation, Vol. 9, No. 5, pages 291-299, October 2004, in the following referred to as [7], P. Koltay, R. Steger, B. Bohl and R. Zengerle, “The dispensing well plate: a novel nanodispenser for the multi-parallel delivery of liquids (DWP Part I)”, Sensors and Actuators A-Physical, Vol. 116, No. 3, pages 483-491, October 2004, in the following referred to as [8], and P. Koltay, J. Kalix and R. Zengerle, “Theoretical evaluation of the dispensing well plate method (DWP Part II)”, Sensors and Actuators A-Physical, Vol. 116, No. 3, pages 472-482, October 2004, in the following referred to as [9].
A fundamentally different form of drop generation is spraying. No single drops are specifically dosed, but a spray cone of drops with an opening angle of frequently more than 10° is generated. Such methods are frequently used for extensive application of coatings. Thereby, particles of a material, e.g. a metal, can at first be transported with a gas jet, and then melted by introducing energy during flight, to be solified again as a layer at the target. Such methods are referred to as thermal spraying, which is described in more detail in DIN EN ISO 2063. Thermal spraying is generally also used for coatings.
CA 2 373 149 A1 describes a method for thermal spraying, where by aerodynamic flow focusing the width of the jet in the target is limited to about one tenth of the diameter of the exit opening of approximately 100 μm. The particles are fused by a laser beam and solidify in the target. The drops applied in that manner can also be thermally post-processed by means of the laser at the target, for example for improving the anchoring of the layer. The method is also commercially offered for three-dimensional printing (company Optomec, brand name M3D). Thereby, the distance of the nozzle to the substrate is approximately 5 mm. Structures up to a height of 150 μm can be built, with layer thicknesses in the range of below 100 nanometer up to several micrometer. It is a disadvantage of this method that no single drops can be dosed therewith, and the feature sizes are too small, for example for metallic mold making.
When developing dosing systems, different requirements for the dosing technology by means of non-contact dosage result. Thereby, the following requirements are fulfilled by many dosing methods:
a) high reproducibility with regard to the dosing position
b) high reproducibility of the drop volume
c) high dosing velocity.
Above that, in particular when dosing liquid media, the following requirements still show need for development:
d) non-contact dosage of relatively highly viscous media
e) non-contact dosage of melted media at high temperatures
f) further reduction of the dosage volume.
One example for non-contact dosage of melted media at high temperatures is described in the expert publication by B. Lemmermeyer, “Ein hochtemperaturbeständiger Einzeltropfenerzeuger für flüssige Metalle”, Technische Universität München, 2006, in the following referred to as [11].
In drop generation by means of flow focusing, two different non-mixable fluids flow in parallel, generally from different nozzles. Thereby, the secondary fluid surrounds the primary fluid. Since the fluids are not mixable, an interface forms between the same. Corresponding to the above described open-jet disintegration, this interface is energetically unstable and generation of single drops of the inner medium is forced. By constrictions of the channel cross section, where the two media stream in parallel, the “stream” of the inner fluid is also constricted. Thereby, the distance between two occurring drops that is proportional to the radius of the beam according to [4; 5] decreases continuously until drop tear-off occurs. Thereby, the drop size is mainly defined by geometry, while the tear-off frequency is determined by the given flow rates. Since constricting the jet is supported by the common flow of media, drops can also be generated from media having a relatively high viscosity, as described in the following expert publications: L. Anna, N. Bontouc and H. A. Stone, “Formation of dispersions using “flow focusing” in microchannels”, Applied Physics Letters, Vol. 82, No. 3, pages 364-366, January 2003, in the following referred to as [12], S. Okushima, T. Nisisako, T. Torii and T. Higuchi, “Controlled production of monodisperse double emulsions by two-step droplet breakup in microfluidic devices”, Langmuir, Vol. 20, No. 23, pages 9.905-9.908, November 2004, in the following referred to as [13], M. Orme, “On the Genesis of Droplet Stream Microspeed Dispersions”, Physics of Fluids A-Fluid Dynamics, Vol. 3, No. 12, pages 2.936-2.947, December 1991, in the following referred to as [14], and S. Suginra, M. Nakajima and M. Seki, “Prediction of droplet diameter for microchannel emulsification”, Langmuir, Vol. 18, No. 10, pages 3.854-3.859, May 2002, in the following referred to as [15].
If the secondary fluid is a liquid, drops or gas bubbles are generated embedded in a liquid phase. Such devices are used for generating emulsions, but also for generating samples on microfluidic chips or generation of foams. Thereby, several stages can be connected in series for obtaining any interlacings of drops [13]. By embedding in liquid, drops generated in that manner cannot be accelerated further directly towards a solid substrate. Additionally, a closed liquid system is necessitated for handling the drops.
Methods where the primary fluid is a liquid and a secondary fluid is a gas are not known.